1. Field of the Invention
This invention relates to an electrode for a lithium ion secondary battery and a lithium ion secondary battery using the same. More particularly, it relates to an electrode structure which can provide such a lithium ion secondary battery that can take an arbitrary shape, for example a thin shape, and has a large energy storage density.
2. Description of the Related Art
There has been a great demand for reduction in size and weight of portable electronic equipment, and its realization relies heavily on improvement of the battery used therein in performance. To meet the demand, development and improvement of a variety of batteries have been proceeding. Characteristics required of batteries include a high voltage, a large energy density, and safety. Lithium ion batteries are secondary batteries that are the most expected of so far developed batteries to achieve a high voltage and a large energy density and will undergo further improvements.
The main part of a lithium ion secondary battery comprises a positive electrode, a negative electrode, and an ionically conducting layer interposed between the electrodes. The lithium ion secondary batteries that have been put to practical use generally employ a positive electrode plate prepared by applying a mixture of a powdered active material, such as a lithium-cobalt oxide, electrically conductive powder, and a binder resin to an aluminum current collector, a negative electrode plate prepared by applying a mixture of a powdered carbonaceous active material and a binder resin to a copper current collector, and an ionically conducting layer made of a porous film of polyethylene, polypropylene, etc. as a separator filled with a lithium ion-containing nonaqueous electrolytic solution.
The improvements on lithium ion secondary batteries in energy density and charge and discharge efficiency greatly rely on the packing density of an active material in each electrode and the utilization of the active material. FIG. 8 schematically illustrates the cross section of an electrode 8 used in a conventional lithium ion secondary battery, which is disclosed, e.g., in JP-A-63-121263 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d). In FIG. 8 reference numeral 1 indicates a powdered active material; 6 an electrically conductive powder; and 7 a current collector. The current collector 7 used in the state-of-the-art electrode, on which an active material is to be applied, is a planar structure, such as foil and net. If the powdered active material 1 alone is applied to the current collector 7, since the amount of the active material in direct contact with the current collector 7 is limited, the current collecting efficiency is not much. Therefore, electrically conductive powder 6, for example, carbon powder should be mixed with the active material powder as stated above.
As described above, a conventional lithium ion secondary battery uses a sheet electrode formed by coating a current collector with a powdered active material mixed with electrically conductive powder for securing sufficient current collecting efficiency. The problem is that the use of the conductive powder reduces the packing density of the active material in the electrode.
Where the battery has a thin shape, it is very likely that the battery is deformed repeatedly by the outer force. In the above-described conventional battery structure, the active material layer tends to be separated from the current collector or broken by the repetition of deformation. Hence, it has been difficult for a thin type battery to maintain durability and safety.
The present invention has been made for the purpose of solving these problems. An object of the present invention is to provide a lithium ion secondary battery which is highly durable, hardly broken, and safe, can take an arbitrary shape, such as a thin shape, and has a large energy density and excellent charge and discharge characteristics.
Another object of the present invention is; to provide an electrode for providing the above-described battery.
The present invention provides an electrode for a lithium ion secondary battery, which is a sheet formed of a mixture comprising a powdered active material and electrically conductive fiber.
A first aspect of the electrode is an electrode of the present invention, which comprises a fiber-containing active material layer of sheet form formed of a mixture comprising a powdered active material and electrically conductive fiber.
A second aspect of the electrode is an electrode of the first aspect, further comprising an active material layer comprising the powdered active material formed thereon.
A third aspect of the electrode is an electrode of the first aspect, wherein the fiber-containing active material layer is formed on an electrically conductive film.
A fourth aspect of the lithium ion secondary battery is a battery of the present invention, wherein at least one of said positive electrode and negative electrode comprises a fiber-containing active material layer of sheet form formed of a mixture comprising a powdered active material and electrically conductive fiber.
A fifth aspect of the lithium ion secondary battery is a battery of the fourth aspect, wherein the electrolyte is a solid electrolyte.
A sixth aspect of the lithium ion secondary battery is a battery of the fourth aspect, wherein the electrolyte is a separator retaining an electrolytic solution.
A seventh aspect of the lithium ion secondary battery is a battery of the present invention, comprising plurality of electrode laminates each of which is composed of a positive electrode, a negative electrode and electrolyte interposed therebetween, wherein at least one of said positive electrode and negative electrode comprises a fiber-containing active material layer of sheet form formed of a mixture comprising a powdered active material and electrically conductive fiber
A eighth aspect of the lithium ion secondary battery is a battery of the seventh aspect, wherein said plurality of electrode laminates are fabricated of a plurality of cut sheets of the electrolyte between which positive electrodes and negative electrodes are alternately interposed.
A ninth aspect of the lithium ion secondary battery is a battery of the seventh aspect, wherein said plurality of electrode laminates are made up of a pair of wound electrolytes between which the positive electrodes and the negative electrodes are alternately interposed.
A tenth aspect of the lithium ion secondary battery is a battery of the seventh aspect, wherein said plurality of electrode laminates are made up of a pair of folded electrolytes between which the positive electrode and the negative electrode are alternately interposed.